1. Field of the Invention
The present invention relates generally to methods and apparatus for transmitting data in cable television network systems. More specifically, the present invention relates to methods and apparatus for locating problematic devices in a cable television plant.
2. Discussion of Related Art
Since the late 1980""s the cable TV industry has been upgrading its signal distribution and transmission infrastructure. In many cable television markets, the infrastructure and topology of cable systems now include fiber optics as part of its signal transmission component. The use of fiber optics has accelerated the pace at which the cable industry has taken advantage of the inherent two-way communication capability of cable systems. The cable industry is now poised to develop reliable and efficient two-way transmission of digital data over its cable lines at speeds orders of magnitude faster than those available through telephone lines, thereby allowing its subscribers to access digital data for uses ranging from Internet access to cable commuting. While cable TV systems have always had the ability to send data downstream, i.e. from a cable TV hub, described below, to cable modems in people""s homes, cable TV systems can now send data upstream, i.e. from individual cable modems to a hub. This new upstream data transmission capability enabled cable companies to use set-top cable boxes and provided subscribers with xe2x80x9cpay-per-viewxe2x80x9d functionality, i.e. a service allowing subscribers to send a signal to the cable system indicating that they want to see a certain program.
FIG. 1 is a block diagram of a two-way hybrid fiber-coaxial (HFC) cable system including cable modems and a network management station. The main distribution component of an HFC cable system is a primary (or secondary) hub 102 which can typically service about 40,000 subscribers or end-users. Hub 102 contains several components of which two, relevant to this discussion, are shown in FIG. 1. One component is a cable modem termination system or, CMTS, 104 needed when transmitting data (sending it downstream to users) and receiving data (receiving upstream data originating from users) using cable modems, shown as boxes 106, 108, 110, and 112. Another component is a fiber transceiver 114 used to convert electrical signals to optical signals for transmission over a fiber optic cable 116. Fiber optic cable 116 can typically run for as long as 100 km and is used to carry data (in one direction) for most of the distance between hub 102 to a neighborhood cable TV plant 117. More specifically, fiber optic cable 116 is a pair of cablesxe2x80x94each one carrying data in one direction. When the data reaches a particular neighborhood cable TV plant 117, a fiber node 118 converts the data so that it can be transmitted as electrical signals over a conventional coaxial cable 120, also referred to as a trunk line. Hub 102 can typically support up to 80 fiber nodes and each fiber node can support up to 500 or more subscribers. Thus, there are normally multiple fiber optic cables emanating from hub 102 to an equal number of fiber nodes. In addition, the number of subscribers as well as fiber capacity is currently increasing due to dense wave-division multiplexing technology. DWDM is a technique for transmitting on more than one wavelength of light on the same fiber.
The primary functions of CMTS 104 are (1) interfacing to a two-way data communications network; (2) providing appropriate media access control or MAC level packet headers (described below) for data on the RF interface of a cable system; and (3) modulating and demodulating the data to and from the cable system.
Cable TV (CTV) taps 122 and 124 are used to distribute a data signal to individual cable modems 106 and 110 (from CTV tap 124) and modems 108 and 112 (from CTV tap 122). Two-way cable TV amplifiers 126 and 128 are used to amplify signals as they are carried over coaxial cable 120. Data can be received by the cable modems shown (each CTV tap can have output cables servicing multiple cable modems) and transmitted back to hub 102. In cable systems, digital data is carried over radio frequency (RF) carrier signals. Cable modems are devices that modulate an RF signal to a digital signal and demodulate a digital signal to an RF signal for transmission over a coaxial cable. This modulation/demodulation is done at two points: by a cable modem at the subscriber""s home and by CMTS 104 located at hub 102. If CMTS 104 receives digital data, for example from the Internet, it converts the digital data to a modulated RF signal which is carried over the fiber and coaxial lines to the subscriber premises. A cable modem then demodulates the RF signal and feeds the digital data to a computer (not shown). On the return path, the operations are reversed. The digital data is fed to the cable modem which converts it to a modulated RF signal. Once CMTS 104 receives the RF signal, it demodulates it and transmits the digital data to an external source.
Data packets are addressed to specific modems or to a hub (if sent upstream) by a MAC layer 130 in CMTS 104 at hub 102 (there is also a MAC addressing component, not shown, in the cable modems that encapsulate data with a header containing the address of the hub when data is being sent upstream). CMTS 104 has a physical layer 134 that is responsible for keeping a list of modem addresses and encapsulating data with appropriate address of its destination. MAC layer 130 receives data packets from a Data Network Interface (not shown) in hub 102. The main purpose of MAC layer 130 is to encapsulate a data packet within a MAC header according to the DOCSIS standard for transmission of data. This standard is currently a draft recommendation (J.isc Annex B) which has been publicly presented to Study Group 9 of the ITU in October 1997, and is known to a person in the cable modem data communication field. MAC layer 130 contains the necessary logic to encapsulate data with the appropriate MAC addresses of the cable modems on the system. Each cable modem on the system has its own MAC address. Whenever a new cable modem is installed, its address must be registered with MAC layer 130. The MAC address is necessary to distinguish data going from the cable modems since all modems share a common upstream path, and so that CMTS 104 knows where to send data. Thus, data packets, regardless of format, must be mapped to a particular MAC address. MAC layer 130 is also responsible for sending out polling messages as part of the link protocol between the CMTS and the cable modems that is necessary to maintain a communication connection between the two.
As mentioned earlier, cable modems can be used to not only receive cable television signals but also digital data and, thus, can provide high-speed access to Internet data or to data from remote computer networks. Because of the increasing usefulness of cable modems for transmitting data over existing cable TV systems, cable modems are proliferating. As they become more prevalent, the burden of maintaining a cable TV plant having a network of cable modems increases. Specifically, the problem of tracking down and isolating a problematic device or group of devices has become increasingly difficult. In response to the increasing need to manage the growing network of cable modems, network management stations are now part of the cable TV system. As shown in FIG. 1, a network management station (NMS) 132 is connected to hub 102, and more specifically to CMTS 104. NMS 132 is located generally at a network operation center, or NOC (not shown in FIG. 1), NMS 132 monitors the CMTS and cable modems using a data communication protocol such as SNMP (Simple Network Management Protocol) and alerts an operator in the event of a failure.
The communication link between the network management station 132 and CMTS 104 has been mostly one-way: commands going from management station 132 to CMTS 104. When a cable modem user is experiencing a problem with a cable modem while, for example, accessing the Internet, the user typically first calls its Internet service provider (ISP). The ISP then calls the cable operator, also referred to as a multiple service operator (MSO), informing it that a particular cable modem is not working. An MSO operator then goes out to the physical site and conducts tests. Often the problem goes away or the problem device cannot be isolated. The MSO then informs the ISO that it could not find anything wrong. This can go on for a while until the MSO can finally locate the problem, which could range from a faulty modem or a bad amplifier along the trunk line. In any event, monitoring of all the cable modems in a given region can quickly become a heavy and unmanageable burden on the NOC, especially as the use of cable modems grows. It can also create dissatisfaction and a sense of unreliability among cable modem users stemming from cable modem plant repair being slow, costly, and inconvenient, e.g. the MSO having to make multiple trips to eventually isolate the problem, sometimes taking up to 10-12 man hours and several interruptions in service.
Therefore, it would be desirable to be able to identify problematic or faulty components in an HFC cable TV system having cable modems supporting two-way transmission of data by using performance data with topological attributes. It would also be desirable to shift the cable plant monitoring task from the network management station to the CMTS, thereby automating the task while not adding to network management traffic by generating any additional messages used solely for testing purposes.
To achieve the foregoing, and in accordance with the purpose of the present invention, methods, systems, and computer-readable media for locating faulty components in a cable modem network configured within a hybrid fiber-coaxial cable television plant are described. In one embodiment of one aspect of the invention, a method of isolating problematic components in a cable modem network includes determining whether a cable modem transmitted a polling message to a central destination during an allotted time interval. A miss count for the cable modem is created reflecting the number of times the cable modem does not transmit a polling message to the central repository during its allotted time. The cable modem can be in one of several states where each state represents a condition of the cable modem. It is then determined whether the cable modem is changing states. This data and the miss count is tabulated in a form that can be analyzed to determine characteristics of the cable television plant. A cable modem entry is inserted into a list of modems experiencing irregular behavior according to the tabulated data relating to the miss count and cable modem state changes.
In another embodiment, the cable modem is informed of a time interval in which the modem can transmit a polling message to the central destination, such as a cable modem termination system. In another embodiment, a cable modem can be in either a hit state, a miss state, or an initial start-up state. In yet another embodiment, the central destination is informed that the cable modem is activated and desires a time interval in which it can transmit a polling message to the central destination. In yet another embodiment a miss count is created by detecting when the cable modem does not transmit a polling message to the central destination during its assigned time interval and counting the number of times the cable modem does not do so. The miss count is reset to zero whenever the cable modem transmits a polling message to the central repository during an assigned time interval.
In another embodiment, a hit count for the cable modem is created reflecting the number of times a polling message is transmitted to the central repository. In yet another embodiment, the cable modem is disconnected from the central destination after a predetermined number of consecutive miss states. The number of times a cable modem changes states and the frequency of the cable modem misses are tabulated and processed through a path analyzing module to locate a faulty component.
In another aspect of the invention, a system for locating faults in a cable television plant having cable modems is described. The system includes a network management console having a fault information processing component. The console conveys fault information in the cable television plant using the fault information processing component. A cable modem termination system having a media access control (MAC) component, prepares data for transmission in the cable television plant and monitors and tabulates fault data related to the cable modems. A memory storage contains data on the cable modems in the form of lists and tables. The cable modems are part of a network of cable modems capable of receiving data from and sending data to the cable modem termination system.
In one embodiment, the network management console has a path analyzing module and a display monitor for displaying output from the module. In yet another embodiment, the MAC component has several state machines for detecting transitions of a cable modem. One of the state machines detects whether a cable modem is in an initial maintenance state. Another state machine detects whether a cable modem has transmitted a polling message to the cable modem termination system during an assigned time interval. In yet another embodiment, the MAC component contains a message processing module that sends maintenance messages to the cable modems and receives polling messages from the cable modems. In yet another embodiment, the memory storage contains a flapping modem list with entries for each modem seen as behaving irregularly and a cable modem table having an entry for each modem in the network. Each entry in the flapping modem list references an entry in the cable modem table.
In another aspect of the invention, a fault locator for use in a cable television plant having a network of cable modems is described. A message processing module send and receives polling messages relating to the network of cable modems. A flapping modem detector checks whether a cable modem changes states more frequently than an acceptable frequency level. If so, the modem is placed in a flapping modem list that stores information relating to the network of cable modems.
In another embodiment, a network management console for correlating fault information and displaying the fault information to a user is connected to the fault locator. In another embodiment, the message processing module sends initial maintenance messages to the cable modems and receives polling messages from the cable modems. In yet another embodiment, the flapping modem detector includes an initial maintenance state machine for detecting when a cable modem has entered an initial maintenance state and a hit/miss state machine for detecting whether a cable modem has transmitted a polling message during an allotted time.
In another aspect of the present invention, a computer-readable medium containing programming instructions for formatting a cable modem list used in a cable modem network fault detection system and having several cable modem entries is described. Each cable modem entry has at least a modem identifier field, one or more polling response fields, and one or more modem state transition fields. The modem identifier field contains a unique identifier for the modem, such as the modem MAC address. The polling response field stores information relating to how often a modem transmits a polling response to a cable modem termination system. The modem state transition field stores information on state transitions of a cable modem.
In one embodiment, one of the polling response fields stores the number of hits made by the modem, a hit indicating that the modem transmitted a polling message during its allotted time interval. In another embodiment, one of the polling response fields stores the number of misses by the modem, a miss indicating that the modem did not transmit a polling message during its allotted time interval. In yet another embodiment, one of the state transition fields stores a count of the number of times a modem transitions into an initial maintenance state. Another one of the state transition fields stores the last time and day a modem changed states.